With the continuous progress of modern communication technology, mobile terminal products have been applied more and more extensively. The antenna plays a more and more important role as a function support foundation and a main component of a mobile terminal.
Along with the rapid development of the third generation mobile communication technology (3G, 3rd Generation), the long term evolution (LTE) band which is as 3G evolution has gradually come into use. At the same time, the second generation mobile communication technology (2G, 2rd Generation) is still widely used. Accordingly, multi communication systems and multi bands coexist.
MIMO is a major breakthrough in smart antenna technology of wireless communications. As a core technology applied in a LTE project, i.e., a new generation wireless communication system, MIMO extends one dimensional smart antenna technology, has really high spectrum efficiency, doubles the communication system capacity without increasing the bandwidth, and enhances the channel reliability.
MIMO refers to a transmitter and a receiver of the signal system, which respectively uses multi transmitting antennas and multi receiving antennas. So the technology is called a multiple-transmitting-antennas-and-multiple-receiving-antennas technology.
At present, the types of antennas applied in mobile terminal products mainly include: monopole antennas, planar inverted-F antennas (PIFAs), loop antennas and so on. Multi-band operations can be achieved by the antennas and technology of coupled feeding, stub addition, slotting, and adjustment matching and so on. However, it is inevitable that the physical space for holding antennas is too large when the size of antennas working in a low-frequency band is too large. While the antenna that works based on a resonant circuit can work on the same frequency band with a relatively smaller size and achieve a high working efficiency. The working frequency bands of LTE include LTE Band 12 (698˜746 MHz) which is lower than the Band of GSM850 (824˜894 MHz), Band 13 (746˜787 MHz) and Band 14 (758˜798 MHz). The antenna which works based on a resonant circuit is a great choice if required to work well with such a small size within such a low frequency band. The space occupied by the antennas can be further reduced if the antennas which work based on resonant circuits (double parallel circuit resonance) are accepted in a high-frequency band. However, the interaction and coupling between antennas have presented a great challenge to small size MIMO antennas. There has been no effective method for improving isolation of MIMO antennas.